This invention relates to an auxiliary-energy-operable hydraulic brake system for automotive vehicles, in which a valve device connected to a pressure medium source and to an unpressurized reservoir and controlling an actuating pressure is operable by an actuating element which, contrary to its direction of actuation, may be pressurized by the actuating pressure or by a force proportional to the actuating pressure.
Auxiliary-energy-type brake systems of the kind referred to are used for generating the required braking performance in heavier vehicles with the aid of relatively small actuating forces. In such cases, a sensitive control of the auxiliary force is required in order to enable an optimal adaptation of the braking action to the respective road and driving conditions.
In a known brake system of the type referred to above (German Printed Patent Application=DE-AS No. 2,602,050) a control slide is provided for the control of the auxiliary force. Said control slide is supported in an axial bore of a booster piston and axially displaceable by means of an actuating device. Depending on the relative position of the control slide, in this system, an axial bore in the control slide and passages arranged in the booster piston and in the control slide permit a working chamber confined by the control-slide-side front face of the booster piston to communicate with either a pressure line connected to an accumulator or with a return line. A relatively wide covering of the passage by the wall surface of the control slide is required in order to be able to close the passage connected to the accumulator in a sufficiently sealed manner in the rest position of the control slide. Further, the control slide has gaskets on both sides of the the passage which serve to contribute to a better sealing of the control slide. The relatively large actuating travel of the control slide and the friction of the gaskets are detrimental to sensitive pressure control for the actuation of the booster piston in particular in case of a low pressure level.
Another control unit for such an auxiliary-energy-operable master cylinder of a vehicular hydraulic brake system (German Patent Specification=DE-PS No. 2,343,882) has a seat valve arranged between the control slide and the accumulator in order to achieve an improved sealing. Upon any actuation of the brake, said seat valve will be opened by a tappet contrary to the accumulator pressure, said tappet being moved by an inclined ramp of the actuating element. In this known control device, the seat valve ensures a good sealing of the pressure port in the rest position. However, the fact of the seat valve's being pushed open against the accumulator pressure leads to an unfavourable responding behaviour of the control unit.
It is thus an object of this invention to reduce the responding force and the actuating travel and to enable a particularly sensitive dosage of the actuating pressure in a hydraulic brake system of the type referred to above.